This is a two way street, unfortunately. They tried to keep Jack2 ABI compatible with Jack1, but failed. Jack2 is missing the metadata API, which breaks compatibility with major tools (Audacity to name one, I believe).

I would like to take this if that would be okay with everyone. I'm enthusiastic about pro audio on Linux and have built a realtime audio stack (Jack - Wine - WineASIO) from raw sources on Gentoo a number of times.